Jesse, this kit is intended to convert a 1993 SVT Cobra from 4 lug 4 wheel disc to 5 lug and 31 spline axles. There was not a SVT Cobra in 1989, additionally all Ford Mustangs were front disc rear drum except for Saleen Mustangs. The kit needed for an 89 is LRS-4234K for the factory 28 spline differential and LRS-4234B-K if the differential has been upgraded to a 31 spline.